Contrary to popular Sunni belief, dogma, assertions and advertisments, the difference between Shia and Sunni IS religous AND political.
It is disingenuous on part of my Sunni brethren to say that the difference is only political. It ignores the beliefs of Shias around the world who ascribe more religous and spiritual meaning to the notion of Imams as their leaders.
What separates Shia from Sunni is the notion of religous leadership and interpretation. Shia's assert that Prophet Mohd.(PBUH) appointed Ali ibn Abu Talib his cousin, son-in-law and the second convert to Islam, as the rightfull religous and political leader of the Muslim ummah after his passing. (Recall that Islam does not make a distinction between din - belief and duniya - world.) This declaration of Imam Ali's role in the Muslim community was made by the Prophet after the first Hajj at Ghadir-Khum. The hadith tradition of the Prophet also gives instances of this declaration:
1."Whoever's maula (lord) I am, so is Ali his maula."
2."I am the house of knowledge and Ali is the doorway."
The religous difference cannot be ignored or swept away. Fundamentally Shias give primary authority for interpretation of the Qur'an and spiritual guidance to the Imam-of-the-time. Thus most shias would have difficulty accepting the fatwas that come out of "religous scholars" from the Sunni tradition. Only the Imam has the authority to provide religous ruling and interpretation.
The Shia's themselves are not homogenous. The two major factions are the the "Seveners" and the "Twelvers," depending on the total number of Imáms they believe were in the true line of succession after Muhammad. The branch which accepts twelve Imáms is currently dominant in Iran. The Twelvers believe that their last Imam has gone into occultation and will come back on the day of judgement. The Seveners consists of two branches, the Ismaili and the Bohra. The Ismaili claim the Aga Khan as the direct decendant of the Prophet and Ali and the 49th Imam after the Prophet.